As long as your starting material or product has a wavelength at which it absorbs strongly then you can use a colorimeter to measure rate of reaction. What you'd need to is prepare your starting materials and add them to a cuvette which will immediately go into the colorimeter. They will react whilst in the colorimeter, at which point you'll want to be measuring a wavelength (you set the wavelength to be detected) which is absorbed by the product but not the starting material. You'll see the absorbance value increasing over time as more product is formed. The reaction should be repeated using different concentrations and the absorption at your monitored wavelength should increase at a different rates. You can also do this by selecting to monitor a wavelength absorbed by the starting material and not the product and measure the rate of decrease of absoroption rather than increase. What you can't do, is monitor a wavelength absorbed by both the starting material and the product (if such a wavelength exists). I'm sure you can see why you can't do that.